The present invention relates to an operation unit for floating point data with a variable exponent-part length.
When dealing with real number data by means of a data processor, it is required that an infinite fraction be included in data with a finite length. A floating point expression is in general employed as a notation relative to the real number. Namely, according to this notation, a bit number (hereinafter referred to as data length) given as an amount of information representing the real number is divided into three categories, viz., sign, exponent and mantissa, the lengths of which are rendered invariable regardless of the real number to be indicated. With this notation adopted, a score of the real number that is expressible in length of a part corresponding to the exponent can be determined; and a significant digit number of real number that is expressible in length of a part corresponding to the mantissa can likewise be determined. This notation, however, involves such a defect that the real number to be used differs in scope according to application objects for which the data processor may be employed and hence it is necessary to create a data processor designed for manipulating the floating point number having a length of the part which expresses a proper exponent in conformity with the application objects.
While on the other hand, another notation has been disclosed wherein there are varied lengths of a part (hereinafter referred to as exponent part) for indicating the exponent thereof in dependence upon the real number to be expressed and of a part (hereinafter referred to as mantissa part) for expressing the mantissa. There is exemplified a data length independent real number value notation based on the double exponent fragmentation (Treatise Bullentin issued by the Information Processing Society of Japan, Vol. 22. No. 6, pp. 521-526 (1981)). The data processor capable of adopting such a notation has an advantage of dealing with the scope of the real number which is virtually needed for all the application objects.
For exemplification, a data processor manipulating the data an exponent part length of which is variable has been disclosed in U.S. patent application Ser. No. 543,426, now U.S. Pat. No. 4,617,641, or Ser. No. 772,696 relative to the application of this assignee. The data processor yields the forementioned advantage, whereas the same processor is degraded by a defect that the significant digit number may be reduced during data processing, a rate of which probably exceeds a expected value. This disadvantage is, it seems, referable to the fact that if a notation in which the overall data length is fixed be employed, the significant digit number (viz., mantissa part length) of the data varies in accordance with the real number value to be displayed, inasmuch as the part length corresponding to the mantissa change according to the real number value to be exhibited. Therefore, in case where various kinds of operations are performed by employing the foregoing device, the significant digit number of data becomes smaller than a predetermined value, so that an error created after completion of every operation does not fall within a desired value. A user of this device can not recognize when the error is produced during operations if the significant digit number is less than anticipated, and it is therefore difficult to grasp the reason why the error becomes bigger than the predetermined value.